Local Code Execution through Argument Injection via dash leading git url parameter in Gemfile

In bundler versions before 2.2.33, when working with untrusted and apparently harmless
Gemfile’s, it is not expected that they lead to execution of external code, unless
that’s explicit in the ruby code inside the Gemfile itself. However, if the Gemfile
includes gem entries that use the git option with invalid, but seemingly harmless,
values with a leading dash, this can be false.

To handle dependencies that come from a Git repository instead of a registry, Bundler
uses various commands, such as git clone. These commands are being constructed using
user input (e.g. the repository URL). When building the commands, Bundler versions
before 2.2.33 correctly avoid Command Injection vulnerabilities by passing an array of
arguments instead of a command string. However, there is the possibility that a user
input starts with a dash (-) and is therefore treated as an optional argument instead
of a positional one. This can lead to Code Execution because some of the commands have
options that can be leveraged to run arbitrary executables.

Since this value comes from the Gemfile file, it can contain any character, including
a leading dash.

Exploitation

To exploit this vulnerability, an attacker has to craft a directory containing a
Gemfile file that declares a dependency that is located in a Git repository. This
dependency has to have a Git URL in the form of -u./payload. This URL will be used to
construct a Git clone command but will be interpreted as the
upload-pack
argument. Then this directory needs to be shared with the victim, who then needs to run
a command that evaluates the Gemfile, such as bundle lock, inside.

Impact

This vulnerability can lead to Arbitrary Code Execution, which could potentially lead
to the takeover of the system. However, as explained above, the exploitability is very
low, because it requires a lot of user interaction. It still could put developers at
risk when dealing with untrusted files in a way they think is safe, because the exploit
still works when the victim tries to make sure nothing can happen, e.g. by manually
reviewing the Gemfile (although they would need the weird URL with a leading dash to
not raise any flags).

This kind of attack vector
has been used in the past
to target security researchers by sending them projects to collaborate on.

Patches

Bundler 2.2.33 has patched this problem by inserting -- as an argument before any
positional arguments to those Git commands that were affected by this issue.

Workarounds

Regardless of whether users can upgrade or not, they should review any untrusted
Gemfile’s before running any bundler commands that may read them, since they
can contain arbitrary ruby code.